Honeycomb heater

Abstract: A honeycomb heater comprising a honeycomb structure (1) which is made of a conductive material and which has a number of passages substantially parallel to a gas flow direction partitioned by partition walls and which has end surfaces on a gas inlet side and a gas outlet side as well as an outer periphery, and at least two electrodes (5) for electrification provided on the honeycomb structure, said honeycomb heater being characterized in that a plurality of slits (10,11) extend in a direction of the passages between the electrodes of the honeycomb structure so that a current flow path may be prolonged, whereby resistance is adjusted, and one or more of a slit forming position, a slit length and a space between each pair of slits are adjusted so that a specific region of the honeycomb heater may be locally heated more quickly as compared with the other region at the time of the electrification. According to the present invention, harmful substances in an exhaust gas discharged from an internal combustion engine, particularly HC generated in large quantities at the time of a cold start can be effectively purified with a low electric power.

Description: BACKGROUND OF THE INVENTION (i) Field of the Invention The present invention relates to an electrical heating type honeycomb heater which can suitably be used in an exhaust gas purification system for effectively purifying harmful components such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO x ) in an exhaust gas exhausted from an internal combustion engine, particularly HC generated in large quantities at the time of an engine start (at a cold start). (ii) Description of the Prior Art Heretofore, the research and development of an automobile exhaust gas purification system for purifying harmful components such as HC, CO and NO x in an exhaust gas of a car have been actively carried out, but particularly in recent years, the purification of these harmful substances at a cold start of a car has been an important technical theme with the intensification of exhaust gas regulations. That is to say, when the temperature of such an exhaust gas as exhausted immediately after the start of an engine is low, a catalyst does not reach its functional temperature, and so the purification ability of the catalyst is poor. In addition, at this time, HC are exhausted in a larger amount than at the time of a steady running. As one means for solving such a technical problem, much attention has been paid to a technique in which an electrical heating type heater is electrified before or simultaneously with the start of the engine to quickly heat the catalyst supported on the heater, the so-called light-off catalyst and main catalyst and the like closely arranged in the rear of the heater up to the functional temperature of the catalysts. For example, EP-A-452125 which has been filed by the present applicant discloses a resistance adjustable type heater in which at least two electrodes for electrification are attached to a honeycomb structure having a number of passages, and a resistance adjusting means comprising slits or the like is provided between the electrodes. However, in the resistance adjustable type heater described in EP-A-452125, the heating rate is low, when the whole heater is heated and a low electric power is applied. Thus, the catalyst supported on the heater cannot reach an ignition temperature in a short period of time, with the result that most of HC generated in large quantities at the time of a cold start is passed through the catalyst without being purified. On the other hand, if a large electric power is applied to quickly heat the heater, a large-capacity battery, heavy cables and a control device are required. EP-A-465184 and EP-A-502731 describe resistance adjusting type heaters comprising honeycomb structures. The structures are intended to provide uniform heating of the heater and have slits for adjusting the resistance. SUMMARY OF THE INVENTION In view of such circumstances, the present invention has been completed. An object of the present invention is to provide a honeycomb heater which can exert a suitable purification performance by the application of a low electric power. The present invention provides a honeycomb heater according to claim 1. As described above, the present invention is concerned with a partial heating type (a local heating type) honeycomb heater in which slits are formed as a means for adjusting resistance, and either or both of a slit position and a space between each pair of slits are adjusted so that a specific region of the honeycomb heater may be locally heated more quickly as compared with the other region at the time of the electrification. The honeycomb heater of the present invention which is used for the purpose of purifying an exhaust gas can usually preferably be utilized in a condition where a catalyst layer is supported on the partition walls of the fundamental honeycomb structure, as described hereinafter. Furthermore, the honeycomb heater is provided with a specific region which can be locally quickly heated (this region will be hereinafter referred to as "the quick heating portion", and the other region will be referred to as "the slow heating portion") as described above, whereby the catalyst layer supported on this region can be promptly ignited with a relatively low electric power, and the heat of reaction obtained by this catalytic reaction accelerates the heating of the whole honeycomb heater, a light-off catalyst and a main catalyst which are usually arranged on the downstream side of this heater. Therefore, when the heater of the present invention is used, harmful substances in the exhaust gas, particularly HC generated in large quantities at the time of a cold start can be effectively purified with the low electric power. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1(a) and 1(b) are illustrative views showing one embodiment of the present invention. Fig.1(a) is a plan view, and Fig.1(b) is a front view. Figs. 2(a) and 2(b) are illustrative views showing one embodiment of the present invention. Fig. 2(a) is a plan view, and Fig. 2(b) is a front view. Fig. 3 is an illustrative view showing one embodiment of the present invention. Figs. 4(a) and 4(b) are illustrative views showing one embodiment of the present invention. Fig.4(a) is a plan view, and Fig.4(b) is a front view. Figs. 5(a) and 5(b) are illustrative views of a conventional known resistance adjustable type heater having slits. Fig.5(a) is a plan view, and Fig.5(b) is a front view. Fig. 6 is a schematic view of a manifold system used to evaluate the performance of the heaters in examples. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a honeycomb structure is made of a conductive material, and examples of the conductive material include metals such as ferrite-containing stainless steel and austenite-containing stainless steel, and ceramics such as perovskite and SiC. However, from the viewpoints of heat resistance and thermal shock resistance, the ferrite-containing stainless steel comprising Fe-Cr-Al is most preferable. Alternatively, partition walls themselves of the honeycomb structure do not have to be made of a conductive material, and for example, an insulating material such as cordierite may be coated with a conductive material such as Pt or cermet by plating or a sputtering process, whereby the coating film of the conductive material can generate heat by electrification. The honeycomb structure may be prepared by winding up a metal foil or by an extrusion technique in accordance with a powder metallurgy method, but in view of the durability of the structure, particularly in order to get rid of a telescoping under severe conditions, it is preferable to use the extrusion technique. No particular restriction is put on the sectional shape (the cell shape) of passages which extend through the honeycomb structure, and any shape of polygons such as rectangle and hexagon, circle and corrugation can be used. However, in view of thermal shock resistance, the flexible cell shape which can relieve thermal stress, for example, the hexagonal shape is preferable. Next, among the honeycomb structures which can be used in the present invention, an embodiment of a process for preparing the metallic honeycomb structure will be described. In the first place, a metal powder material having a desired composition is prepared from, for example, an Fe powder, an Al powder, a Cr powder or a powder of an alloy thereof. Next, the thus prepared metal powder material is mixed with an organic binder such as methyl cellulose or polyvinyl alcohol and water, and the resulting mixture is then extruded into a desired honeycomb shape. In the mixing step of mixing the metal powder material with the organic binder and water, it is preferable to mix the metal powder with an antioxidant such as oleic acid prior to the addition of water. Alternatively, it is also preferable to use the metal powder beforehand subjected to an anti-oxidizing treatment. Next, the extruded honeycomb is fired at a temperature of 1,000 to 1,400°C under a non-oxidizing atmosphere. In this case, the firing is preferably carried out under the non-oxidizing atmosphere containing hydrogen, because Fe or the like can be utilized as a catalyst to decompose and remove the organic binder, thereby obtaining a good sintered body. If the firing temperature is less than 1,000°C, the molded article is not sintered, and if it is more than 1,400°C, the deformed sintered body is inconveniently obtained. Next, the surfaces of the partition walls and pores of the obtained sintered body are coated with a heat-resistant metal oxide, if desired. Preferable methods for coating the same with the heat-resistant metal oxide are as follows. (1) The metal honeycomb structure is thermally treated at 700 to 1,100°C in an oxidizing atmosphere. (2) The surfaces of the partition walls and pores of the sintered body are plated with aluminum or the like (e.g., a gaseous plating), followed by a heat treatment at 700 to 1,100°C in the oxidizing atmosphere. (3) The sintered body is immersed in a metal hot melt of aluminum or the like, followed by a heat treatment at 700 to 1,100°C in the oxidizing atmosphere. (4) The surfaces of the partition walls and pores of the sintered body are coated with an alumina sol, followed by a heat treatment at 700 to 1,100°C in the oxidizing atmosphere. The temperature of the heat treatment is preferably in the range of 900 to 1,100°C from the viewpoints of heat resistance and oxidation resistance. The honeycomb structure is provided with at least two electrodes for electrification. Here, the electrodes include an electrode on an earth side. The electrodes are connected to a power source such as a battery or a capacitor via a switch and a control device so that they may be electrified, and they can be attached to an outer or an inner periphery of the honeycomb structure. It is preferable that a catalyst layer containing a catalyst-activating substance is supported on the partition walls of the honeycomb structure so as to cover them. The catalyst layer supported on the partition walls of the honeycomb structure comprises a carrier having a large surface area and the catalyst-activating substance supported on the carrier. Typical examples of the carrier having the large surface area include γ-Al 2 O 3 , TiO 2 , SiO 2 -Al 2 O 3 and perovskite. Typical examples of the catalyst-activating substance include noble metals such as Pt, Pd and Rh, and base metals such as Cu, Ni, Cr and Co. Among others, preferable is an example in which one or more noble metals of Pt, Pd and Rh are supported on γ-Al 2 O 3 . The thus constituted honeycomb heater of the present invention has end surfaces on gas inlet and gas outlet sides, and an exhaust gas flows from the gas inlet side end surface to the gas outlet side end surface via the passages. The honeycomb heater is usually held preferably in an insulating state in a metallic can, and it is arranged so that the exhaust gas may be introduced into the honeycomb heater in the can through its gas inlet side end surface. The honeycomb heater is heated by the electrification, but at this time, it is necessary that slits are formed so that a specific region of the honeycomb heater, preferably a specific region including at least a gas inlet side end surface neighborhood may be locally heated more quickly as compared with the other region of the heater. The reason why the quick heating region is preferably constituted so as to include at least the gas inlet side end surface neighborhood is that, as described above, the heat of reaction generated by locally quickly heating the upstream portion of the honeycomb heater can transfer to the lower portion of the honeycomb heater in turn to heat the catalyst layer on the downstream side of the honeycomb heater or the light-off catalyst or the like arranged in the downstream portion of the honeycomb heater. Conversely, if a gas outlet side end surface neighborhood alone is heated by the electrification, the gas inlet side end surface neighborhood is not heated, and so the same functions as a heat sink. In consequence, the power to be applied and a purification efficiency are unsatisfactory. It is also one of suitable embodiments to uniformly (locally as seen from the section of a gas passage) and quickly heat the honeycomb heater from the gas inlet side end surface to the gas outlet side end surface in a gas flow direction. In this case, a large amount of the heat of reaction generated by the catalyst on the heater is useful to early activate the light-off catalyst or the main catalyst which is usually arranged immediately in the rear of the heater. In the present invention, the phrase "locally quickly heated" means that one or more quick heating portions and one or more slow heating portions (inclusive of non-heating portions) are present in the heater, and the honeycomb heater is heated so that the slow heating portions may be lower than a catalyst ignition temperature by 50°C or more, preferably 100°C or more, when the quick heating portions have reached the catalyst ignition temperature. If the temperature difference is less than 50°C, the heater is wholly heated, so that the intended effect of the power reduction is poor. If it is more than 100°C, the suitable purification performance can be exerted by the application of the relatively low electric power. Moreover, the phrase "catalyst ignition temperature" means a temperature at which the reaction intermittently occurs without the extinction of the catalyst, and the aim of the catalyst ignition temperature is 350°C or more. The catalyst deteriorates when used for a long period of time, and in the case of the local heating type heater, there is a problem of a heat drawing of the slow heating portions. Therefore, a temperature of 500°C or more can be surely considered to be the catalyst ignition temperature. Next, some embodiments of the present invention will be described with reference to drawings. In a first embodiment of the present invention, as shown in Figs. 1(a) and 1(b), the outer periphery of a honeycomb structure 1 is divided into a first semicircular outer periphery a-a' and a second semicircular outer periphery b-b' by two electrodes 5 so that these peripheries may face to each other, and at least two first slits 10 and at least two second slits 11 are alternately arranged except slits in the vicinity of the electrodes. Here, the first slits 10 cut the first semicircular outer periphery a-a' but do not cut the second semicircular outer periphery b-b', and the second slits 11 cut the second semicircular outer periphery b-b' but do not cut the first semicircular outer periphery a-a'. The number of each of the alternately arranged first slits 10 and second slits 11 is 2 or more, and for example, the number may be 3. However, even if the number of the slits is meaninglessly increased, the effect of the present invention does not substantially change. In this constitution, the regions put between the first slits 10 and the second slits 11 allow current to flow therethrough, but on the other hand, the regions put between the first slits 10 or between the second slits 11 scarcely allow current to flow therethrough. In consequence, belt-like quick heating portions are formed in the regions put between the first slits 10 and the second slits 11, as shown by hatches in the plan view of Fig. 1 (a) and the front view of Fig. 1 (b). Figs. 5(a) and 5(b) show a conventional known resistance adjustable type heater having slits which is described as an example in EP-A-452125 already filed by the present applicant. In this known heater, the outer periphery of a honeycomb structure 1 is divided into a first semicircular outer periphery a-a' and a second semicircular outer periphery b-b' by two electrodes 5 so that these peripheries may face to each other, and first slits 60 and second slits 61 are alternately arranged at substantially equal intervals. Here, the first slits 60 cut the first semicircular outer periphery a-a' but do not cut the second semicircular outer periphery b-b', and the second slits 61 cut the second semicircular outer periphery b-b' but do not cut the first semicircular outer periphery a-a'. In such a constitution, current flows through a region put between the pair of slits, so that the whole heater is uniformly heated as shown by hatches in a plan view of Fig. 5 (a) and a front view of Fig. 5 (b). Therefore, it is difficult to achieve quick heating with a low electric power. In a second embodiment of the present invention, as shown in Figs. 2(a) and 2(b), the outer periphery of a honeycomb structure 1 is divided into a first semicircular outer periphery a-a' and a second semicircular outer periphery b-b' by two electrodes 5 so that these peripheries may face to each other, and first slits 12 and second slits 13 are alternately arranged so that spaces between the first slits 12 and the second slits 13 may be ununiformed. Here, the first slits 12 cut the first semicircular outer periphery a-a' but do not cut the second semicircular outer periphery b-b', and the second slits 13 cut the second semicircular outer periphery b-b' but do not cut the first semicircular outer periphery a-a'. That is to say, this embodiment is the same as a conventional known heater shown in Figs. 5(a) and 5(b) in that the first slits 12 and the second slits 13 are alternately arranged, but the spaces between the first slits 12 and the second slits 13 may be not uniform and thus two or more kinds of slit spaces having different widths may be formed. Consequently, in the regions having the relatively narrow spaces, the sectional area of a current flow path is smaller (a current density is higher) than in the regions having the wide spaces, and so the regions having the relatively narrow spaces are predominantly heated. Therefore, as shown by hatches in a plan view of Fig. 2(a) and a front view of Fig. 2(b), belt-like quick heating portions are formed in the regions having the relatively narrow spaces between the slits. In the embodiment shown in Figs. 2(a) and 2(b), the regions having the narrow spaces and the regions having the wide spaces are alternately arranged, but instead of such an alternate arrangement, these regions can also be optionally combined to quickly heat the specific regions. Moreover, it is also possible to optionally change each space between the first slit 12 and the second slit 13. In a third embodiment of the present invention, as shown in Fig. 3, the outer periphery of a honeycomb structure 1 is divided into a first semicircular outer periphery a-a' and a second semicircular outer periphery b-b' by two electrodes 5 so that these peripheries may face to each other, and first slits 14 and second slits 15 are alternately arranged so that spaces between the first slits 14 and the second slits 15 may be ununiformed, so that regions having relatively narrow spaces put between the first slits and the second slits are scatteringly present, and so that the sectional area of these regions may be relatively small. Here, the first slits 14 cut the first semicircular outer periphery a-a' but do not cut the second semicircular outer periphery b-b', and the second slits 15 cut the second semicircular outer periphery b-b' but do not cut the first semicircular outer periphery a-a'. That is to say, this third embodiment is the same as the second embodiment in that the spaces between the first slits 14 and the second slits 15 are not uniform to form two or more kinds of slit spaces having different widths, but in the third embodiment, the length of the first slits 14 and the second slits 15 is adjusted to form the relatively narrow spaces, and the regions put between these slits are scatteringly present, so that the sectional area of these regions is relatively small. Therefore, in contrast to the second embodiment in which the belt-like quick heating portions are formed, the third embodiment provides the scattered quick heating portions, as shown by hatches in a plan view of Fig. 3. In this embodiment, the spaces between the first slits 14 and the second slits 15 can be optionally changed to scatteringly arrange the quick heating portions at optional positions, as in the above-mentioned second embodiment. The various slits in the embodiments 1 to 3 described above can be used in an optional combination. For example, Figs. 4(a) and 4(b) show the combination of the concept of the first embodiment and the concept of the second embodiment, and the outer periphery of a honeycomb structure 1 is divided into a first semicircular outer periphery a-a' and a second semicircular outer periphery b-b' by two electrodes 5 so that these peripheries may face to each other, and three first slits 26 and three second slits 27 are alternately arranged except slits in the vicinity of the electrodes so that spaces between the first slits 26 and the second slits 27 may be narrower than spaces between the first slits 26 or between the second slits 27. Here, the first slits 26 cut the first semicircular outer periphery a-a' but do not cut the second semicircular outer periphery b-b', and the second slits 27 cut the second semicircular outer periphery b-b' but do not cut the first semicircular outer periphery a-a'. The above-mentioned embodiments are merely exemplary, and the present invention can take various constitutions other than the above-mentioned embodiments, so long as specific regions of the honeycomb heater are constituted so as to be locally heated more quickly as compared with the other region, and the embodiments of the present invention can be combined with conventional known various resistance adjusting means. In the present invention, the area of the quick heating portion is preferably in the range of 5 to 50% of the sectional area of the gas passage. If the area of the quick heating portion is less than 5% of the sectional area of the gas passage, a large amount of the gas may pass through the slow heating portion without being purified, and heat of reaction by the catalyst in the quick heating portion is not so expected, so that a purification efficiency lowers. On the other hand, if it is more than 50%, the weight of the quick heating portion increases, and so it is necessary to increase the electric power to be applied, with the result that the effect of reducing the electric power decreases. The preferable ratio of the area of the quick heating portion to the sectional area of the gas passage is in the range of 10 to 40%, whereby there can be reduced 30% or more of the electric power required to heat a whole heating type heater in which most of the section of the gas passage is uniformly heated, and even by such a reduced electric power, a similar purification efficiency can be obtained. In this case, the area of the quick heating portions can easily be calculated, for example, in the following way. That is to say, in the first place, the heating properties of the quick heating portions are evaluated in still air by the use of a thermovision. Next, in order to evaluate this honeycomb heater by an actual driving test, many thermocouples are attached to the honeycomb heater, and temperatures at many positions in the quick heating portions are measured, electrifying and heating the quick heating portions in accordance with an FTP (federal test procedure) test. Thus, the area of the quick heating portions can be determined from a temperature distribution on the thermovision and the actual heating properties of the heater. The honeycomb heater of the present invention aims at a low-power type, and so the preparation of the relatively small heater is preferable. Preferably, the volume of the gas passage portion is set to about 30 to 300 cc. If the volume of the gas passage portion is less than 30 cc, the mechanical strength of the heater may be poor and the effective area of the catalyst on the heater limited, so that the heat of reaction obtained therefrom may be insufficient and the desired purifying performance may not be obtained. Conversely, if it is more than 300 cc, the honeycomb heater becomes a local heating type, but the weight of the heater increases, so that the application of an increased electric power is required. The preferable volume of the heater is in the range of 40 to 150 cc, and the effective purification can be achieved with a low electric power of less than 3 kW, depending upon a position where the heater is arranged. "The low electric power" referred to herein means that watt × time is as low as possible, but the long-time electrification at a low watt is more preferable than the short-time electrification at a high watt. That is to say, for the electrification at the high watt, a high voltage and/or a large current is necessary. In the case of the high voltage, a complicated power source system and control system are required, and in the case of the large current, extremely thick cables and the like are required. Thus, in the case of a gasoline car, the battery power source and the current are preferably adjusted to be 12 V and 200 A or less, respectively. In this case, a power of 2 kW or less is to be applied to the heater, in view of internal resistances of the battery and the control system. As the power source, an alternater and an ultracapacitor can be preferably used in addition to the battery. The length of the honeycomb heater is usually in the range of 6 to 40 mm, depending upon the sectional area of the honeycomb heater. If the length of the honeycomb heater is less than 6 mm, deformation may occur due to creep at the time of a high temperature, and if it is more than 40 mm, the honeycomb heater may deform due to thermal stress generated in the heater. Preferably, the length of the quick heating portion in a gas flow direction is 25 mm or less from the exhaust gas inlet end surface, and it is equal to or less than the length of the heater. If the length of the quick heating portion is more than 25 mm, the heat capacity of the quick heating portion increases, so that the amount of the electric power to be applied may increase inconveniently. The thickness of the partition walls of the honeycomb heater is preferably in the range of about 40 to 300 µm, and the cell density is preferably in the range of 150 to 600 cells/inch 2 (23 to 93 cells/cm 2 ). In order to efficiently purify harmful substances in the exhaust gas, particularly HC generated in large quantities at the time of the cold start by the use of the honeycomb heater of the present invention described above, the electrification is preferably carried out so that the quick heating portion may reach a catalyst ignition temperature within 20 seconds from the cold start. The electrification of the heater is carried out in an optional manner, for example, by a preheating means for starting the electrification before engine crank, a postheating means for starting the electrification immediately after the engine crank, or a combination of the preheating means and the postheating means, and this electrification can be done continuously or intermittently. In view of the simplicity of the control system, the postheating means is preferable, and it is also preferable to continuously apply the power once at the time of the cold start. It is preferred that the temperature of the quick heating portion reaches 500°C or more as early as possible, and in the Bag 1 of an FTP test, it is one aim that the temperature is reached within 20 seconds (at the time of idling). At this time, the upper limit of the temperature of the quick heating portion is about 1,000°C, and so the temperature of the quick heating portion is preferably 1,000°C or less. If the temperature of the quick heating portion is in excess of 1,000°C, there is a fear that the honeycomb heater is deformed or broken owing to thermal stress generated by a temperature difference between the quick heating portion and the slow heating portion. In the case that the exhaust gas purification system is constituted of such a honeycomb heater as in the present invention, the so-called light-off catalyst or main catalyst is usually arranged on the downstream side of the honeycomb heater. Since the honeycomb heater is operated with the low power, its size is relatively small, and so the honeycomb heater itself is insufficient to purify all the HC exhausted at the cold start. Therefore, the relatively small (about 0.2 to 1.2 ℓ) light-off catalyst is arranged on the downstream side of the honeycomb heater. In addition, for the purpose of improving the purification performance at a steady drive (e.g., Bag 2), the so-called main catalyst having a relatively large volume (about 1 to 3 ℓ) is arranged on the downstream side of the light-off catalyst. As another constitution, for example, the relatively small (about 0.2 to 1.2 ℓ) light-off catalyst may be arranged on the most upstream side of the exhaust gas flow passage, and the honeycomb heater may follow, and the light-off catalyst and the main catalyst may be further arranged on the downstream side of the honeycomb heater. In this case, the application of a little more power to the honeycomb heater is required as compared with the constitution in which the honeycomb heater is arranged on the most upstream side, but the most upstream light-off catalyst functions as a thermal shock absorber, so that it is not necessary any more to be anxious about the durability of the honeycomb heater. The above-mentioned "light-off catalyst" and "main catalyst" are common names, and in short, they are catalysts having a catalytic function of capable of purifying the exhaust gas discharged from an internal combustion engine. In general, as these catalysts, there can be used ceramic or metallic honeycomb structures on which a catalyst activating substance having the catalytic function is supported in the form of a catalyst layer. The honeycomb heater is preferably arranged at the so-called manifold position in the vicinity of an engine exhaust port so as to utilize exhaust heat. In this case, the honeycomb heater is exposed to particularly severe exhaust gas conditions, and so it is preferable to use the honeycomb heater prepared by an extrusion method, as described above. When arranged at the manifold position, the honeycomb heater exerts the suitable purification performance with an electric power of 1 kW or less. At the time of the cold start, a car is usually driven in a rich fuel state, but in such a state, the HC purification performance of the honeycomb heater is insufficient, even if the heater is quickly heated. Thus, the introduction of secondary air is effective, or it is also preferable that after engine crank, the amounts of the fuel and air are adjusted to shift the composition of the exhaust gas to a nearly stoichiometric amount or a slightly lean side. As described above, according to the present invention, harmful substances in an exhaust gas discharged from an internal combustion engine, particularly HC generated in large quantities at the time of a cold start can be effectively purified with a low electric power. Next, the present invention will be described in more detail with reference to examples, but the scope of the present invention should not be limited to these examples. Example 1 A honeycomb heater of the above-mentioned first embodiment was prepared in accordance with the following preparation procedure. In this case, the number of each of first slits and second slits was 5 (inclusive of slits in the vicinity of electrodes), and the two first slits and the two second slits were arranged so as to alternate with each other. Furthermore, the number of the cells between the splits was 6, and the distance from the tip of each slit to the outer periphery (the distance on a line extended from the slit) was set to 7 mm. [Preparation method of honeycomb heater] An Fe powder, a Cr-30 Al powder (wt%), an Fe-50 Al powder (wt%), an Fe-20 B powder (wt%) and a Y 2 O 3 powder, all having an average particle diameter of 44 µm or less were mixed so as to obtain a composition of Fe-16 Cr-8 Al-0.05 B-0.5 Y 2 O 3 . Next, 4 g of methyl cellulose as an organic binder and 1 g of oleic acid as an antioxidant were added to and mixed with 100 g of the resulting mixture. After the preparation of such a clay, it was then extruded into a cylindrical honeycomb article. This honeycomb article was dried at 90°C for 16 hours in the atmosphere, and then held at 1,325°C for 2 hours to sinter the article. Next, the article was subjected to a heat treatment at 1,150°C for 30 minutes in air. In accordance with the above-mentioned procedure, a honeycomb structure was obtained which had an outer diameter of 93 mm, a thickness of 12 mm, a partition wall thickness of 0.1 mm and a hexagonal cell density of 450 cells/inch 2 . The porosity of this honeycomb structure was 2% (substantially non-porous), and a protective layer of an Al 2 O 3 component was formed on the surface of the honeycomb structure. Slits were formed in this honeycomb structure as described above to prepare quick heating portions. After the formation of the slits, a mixture of γ-Al 2 O 3 and CeO 2 was prepared so that a weight ratio of γ-Al 2 O 3 :CeO 2 might be 70:30, and water and a trace amount of nitric acid were then added to the mixture. Afterward, the mixture was ground by a wet method to prepare a carrier slurry, and a wash coat was then formed on the honeycomb structure in accordance with a dipping method by the use of this carrier slurry. Next, this wash coat was dried, and then fired at a temperature of 500°C to cover the honeycomb structure with γ-Al 2 O 3 and CeO 2 . The thus treated honeycomb structure was immersed in an aqueous solution comprising palladium nitrate, chloroplatinic acid and rhodium nitrate for about 20 minutes so that a molar ratio of Pd:Pt:Rh might be 6:5:1 and so that the total amount of the supported components might be 80 g/ft 3 , whereby a catalyst composition was supported on the honeycomb structure. Next, electrode bolts were welded at two positions on the outer periphery of the honeycomb structure to prepare a honeycomb heater. This honeycomb heater was held in a metallic can by fixing the outer periphery of the honeycomb heater via an insulating material. Example 2 A honeycomb heater of the above-mentioned second embodiment was prepared in accordance with the same preparation procedure as in Example 1. In this example, the number of each of first slits and second slits was 5, and they were arranged so as to alternate narrow spaces and wide spaces with each other. In this case, the number of cells between the slits defining the narrow spaces was 4, and the number of cells between the slits defining the wide spaces was 8. The distance from the tip of each slit to the outer periphery (the distance on a line extended from the slit) was set to 7 mm. Example 3 A honeycomb heater of the above-mentioned third embodiment was prepared in accordance with the same preparation procedure as in Example 1. In this example, 5 quick heating portions were formed by adjusting the length of first slits and second slits so that they might be scatteringly present. In this case, the number of cells between the slits defining the narrow spaces which became the quick heating portions was 3, and the number of cells between the slits defining the wide spaces was 9. The regions put between the slits for defining the narrow spaces which became the quick heating portions had 3 mm width × 15 mm length. Comparative Example 1 In accordance with the same procedure as in Example 1, a conventional known resistance adjustable type heater was prepared in which first slits 60 and second slits 61 were alternately formed from mutually faced semicircular outer peripheries at substantially equal intervals, as shown in Figs. 16(a) and 16(b). In this case, the number of each of the first slits and the second slits was 5. Furthermore, the number of the cells between each pair of slits was 6, and the distance from the tip of each slit to the outer periphery (the distance on a line extended from the slit) was set to 7 mm. [FTP Test] The thus obtained honeycomb heaters in Examples 1 to 10 and Comparative Example were used, and a light-off catalyst having a volume of 0.9 ℓ was arranged immediately on the downstream side of each heater. Next, a main catalyst having a volume of 1.7 ℓ was arranged on the further downstream side of the light-off catalyst to constitute an exhaust system shown in Fig. 17. In the drawings, reference numeral 100 was a honeycomb heater, numeral 101 was a light-off catalyst, 102 was a main catalyst, and 103 was an engine. This engine was an L4 type having a displacement volume of 2,000 cc. The honeycomb heater, the light-off catalyst and the main catalyst were all exposed to a stoichiometric exhaust gas having an inlet temperature of 850°C, and acceleratedly consumed in a mode in which a fuel was cut for 5 seconds every one minute to deteriorate them. The thus treated honeycomb heater, light-off catalyst and main catalyst were then used. Bag emission was measured in accordance with FTP (federal test procedure) to evaluate cold start properties of the honeycomb heaters of examples. During this measurement, secondary air was introduced into the exhaust system from the upstream side of the honeycomb heater at 120 ℓ/min for 100 seconds from the start of the engine. Furthermore, electrification was given by the use of a constant-voltage generator, and voltage was regulated so as to obtain 1 kW value. An electrification time was 30 seconds after engine crank. FTP Performance (HC emission) Bag 1A (g) Bag 1 (g/mile) Example 1 0.78 0.19 Example 2 0.75 0.17 Example 3 0.80 0.21 Comp. Ex. 0.86 0.25

Claims: A honeycomb heater comprising a honeycomb structure (1) which is made of a conductive material and has a gas passage area formed by a plurality of passages substantially parallel to the gas flow direction and separated from each other by partition walls and which has an end surface (6) at a gas inlet side, and an end surface (7) at a gas outlet side as well as an outer periphery, and at least two electrodes (5) for application of an electric current provided on the honeycomb structure (1), wherein a plurality of parallel slits (10,11,12,13,14,15) are provided so as to extend in the direction of the passages and in a direction transverse to the direction between the electrodes (5) of the honeycomb structure (1) so that the current flow path between the electrodes (5) is prolonged, whereby resistance is adjusted, said slits (10,11,12,13,14,15) being arranged such that first regions formed between pairs of parallel slits and having a relatively high heating rate when current is applied are spaced apart by a second region or regions also formed between pairs of parallel slits and not having said relatively high heating rate. A honeycomb heater according to claim 1 wherein the outer periphery of the honeycomb structure (1) is divided into a first outer periphery region and a second outer periphery region by the two electrodes (5), and said parallel slits (10,11) are arranged so that an adjacent pair (10) thereof which both cut the first outer periphery region but not the second outer periphery region are succeeded, in the direct direction between the electrodes (5), by a further adjacent pair (11) thereof which both cut the second outer periphery region but not the first outer periphery region. A honeycomb heater according to claim 1 wherein the outer periphery of the honeycomb structure (1) is divided into a first outer periphery region and a second outer periphery region by the two electrodes (5) and first ones of said slits (12) cutting the first outer periphery but not cutting the second outer periphery and second ones of said slits (13) cutting the second outer periphery but not cutting the first outer periphery are alternately arranged in sequence in the direct direction between the electrodes (5) and the spacings between the first slits (12) and the second slits (13) are non-uniform. A honeycomb heater according to claim 1 wherein the outer periphery of the honeycomb structure is divided into a first outer periphery region and a second outer periphery region by the two electrodes (5), and first ones of said slits (14) cutting the first outer periphery region but not cutting the second outer periphery region and second ones of said slits (15) cutting the second outer periphery region but not cutting the first outer periphery region are alternately arranged in sequence in the direction between the electrodes (5), and the spacings between the first slits (14) and the second slits (15) are non-uniform, and said first and second slits (14,15) overlap along a short portion of their length such that said regions of relatively high heating rate are formed at the region of overlap between adjacent first and second slits (14,15) with narrow spacings therebetween. A honeycomb heater according to any one of claims 1 to 4 wherein the thickness of the partition walls of the honeycomb structure (1) is varied, in at least part of the honeycomb structure (1), to adjust electrical resistance. A honeycomb heater according to any one of claims 1 to 5 wherein the cell density of the honeycomb structure (1) is varied, in at least part of the honeycomb structure (1) to adjust electrical resistance. A honeycomb heater according to any one of claims 1 to 6 wherein regions are formed in which the partition walls in the honeycomb structure (1) are partially eliminated. A honeycomb heater according to any one of the preceding claims wherein a catalyst layer is supported on the partition walls of the honeycomb structure (1). A honeycomb heater according to any one of the preceding claims wherein the area of said region or regions which is heated at a relatively high rate is in the range of 5 to 50% of the section of the gas passage. A honeycomb heater according to any one of the preceding claims wherein said region or regions which are heated at a relatively high rate includes at least the end surface (6) at the gas inlet side.

Inventors: ABE FUMIO, HASHIMOTO SHIGHARU, KONDO TOMOHARU, ABE, FUMIO, HASHIMOTO, SHIGHARU, KONDO, TOMOHARU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1994
Main classification: B01J35
IPC codes: H05B3, B01J35, F01N3